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Abstract

Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale. However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation. We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links. From these measurements, we predict the transfer stability that can be achieved over transoceanic links. We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field. Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.

Fig. 2. Sketch of our experiment: ultrastable radiation was generated by locking a fiber laser (laser 1) to a Fabry–Perot cavity (FPC), then sent from University of Malta to the landing station in Malta via a Doppler-stabilized link and here regenerated by a second fiber laser (laser 2). The regenerated light was sent over the submarine link and compared to the local one to detect the fiber-induced noise after a round trip. FM, Faraday mirrors; PD, photodiodes; AOM, acousto-optic modulators (frequency shifters); EDFA, erbium-doped fiber amplifier (used on L2 only).

Fig. 4. Frequency instability of the submarine fiber over 24 h (black, squares). Also shown is the instability of the short link composed of the 2×25m patch cords connecting our apparatus to the submarine cables (red circles).

Fig. 6. PSD of the submarine fiber noise on a submarine telecom optical scale (L1, black) and on an optical cable deployed inside a high-voltage submarine power line, also including 19 km of onshore fiber (L2, red).

Fig. 8. Phase noise and Allan deviation for various configurations. Gray and light gray: measured values for the L1 and L2 testbeds, established over a ∼100km looped fiber; black line: mathematical model for the observed noise; green line: extrapolated noise of a free-running 7000 km transoceanic fiber; orange and red lines: expected behavior of a two-way frequency comparison over a 7000 km transoceanic fiber, established over a single fiber or a fiber pair, respectively. The estimated spectra in the graph are plotted only for Fourier frequency f<3Hz, where the low-frequency approximation is valid.

Fig. 9. Black line, “FIBER”: the PSD of the optical phase; blue line, “IV.HPAC” the displacement of a seismometer located on the shore in Pachino, Sicily (IV.HPAC); red line, “MN.WDD” the displacement of a seismometer located in Malta (MN.WDD). The spectra were calculated in the same time window and have been re-normalized for better visualization.